I just purchased the game and now finding out I cannot invert the mouse or turn down the music, I am really frustrated. Sounds lame, but I spent over an hour looking for a new Early Access to try and like the OP, I too cannot play the game without inverted mouse.
I just found in the DefaultInput.ini file these lines:
+AxisConfig=(AxisKeyName="MouseX",AxisProperties=(DeadZone=0.000000,Sensitivity=0.070000,Exponent=1.000000,bInvert=False))
+AxisConfig=(AxisKeyName="MouseY",AxisProperties=(DeadZone=0.000000,Sensitivity=0.070000,Exponent=1.000000,bInvert=False))
I assume changing them to true will fix the problem? I will test and get back to you
Confirmed changing the False to True does fix the inverted mouse!
See my post below I fixed it.